In keeping with the recent advanced information-based society, application systems including an information-communication apparatus that utilize a radio-frequency range, such as a microwave band ranging from 1 to 30 GHz and a millimeter-wave band ranging from 30 to 300 GHz, have been proposed for high-speed transmission of large-volume data. Moreover, millimeter wave-using systems such as a radar device for measuring inter-vehicle distance have also been proposed to date.
In order to attain a radio-frequency circuit for use in a radiofrequency range-using system, for example, a waveguide is formed in a dielectric substrate, and an electronic component is mounted thereon. The electronic component is composed of one or a plurality of various semiconductor elements such as a diode and a transistor. A circuit formed in the dielectric substrate is connected to a DC (direct current) power source for applying bias voltage, bias current, or the like to operate the semiconductor element.
If, in the semiconductor element, an unintended input-output terminal receives application of DC voltage, or DC voltage beyond the limit of the rating is applied, the semiconductor element may suffer from performance deterioration or a breakdown. In order to prevent such an unnecessary DC voltage application, under the conventional technology, a capacitor is provided as a direct-current blocking circuit. The examples of such a capacitor include an interdigital capacitor which is formed in a microstrip line, a gap-type capacitor which is constructed by forming a gap in a microstrip line, and a multilayer ceramic capacitor (for example, refer to “Monolithic Microwave Integrated Circuit (MMIC)” written by Masayoshi Aikawa and four other authors (published by the Institute of Electronics, Information and Communication Engineers (IEICE), January 1997 issue, p. 47 to p. 48)).
The interdigital capacitor, as well as the gap-type capacitor, is required to allow passage of radio-frequency signals traveling through the microstrip line with a low loss while blocking a direct current component.
In a case where a direct-current blacking circuit is formed on a ceramic substrate having a specific permittivity of 11 or below, there is a need to provide a reactance component corresponding to the frequency of a radio-frequency signal intended to be transmitted. In a planar circuit such as a microstrip line, a desired reactance component may be formed by making proper adjustment to line-to-line width and length or by adopting a multilayer ceramic capacitor as a direct-current blocking circuit. However, in the fabrication method for transmission lines based on conductor printing and co-firing techniques, since the lower limits of line-to-line width and length are determined in accordance with design rules, it is impossible to obtain the desired reactance component required to transmit radio-frequency signals, especially those in a millimeter-wave band or the like. Accordingly, there arises a large reflection from the direct-current blocking circuit with consequent heavy transmission loss.
Furthermore, in a case where a multilayer ceramic capacitor is used for the direct-current blocking circuit, an inductance component of a metal electrode is so great that its influence is not negligible. This makes it impossible to establish the desired reactance component with the consequence that the reflection of a passing radio-frequency signal becomes so great that the transmission characteristics may be deteriorated.